A temperature sensor does not provide an immediate output corresponding to a change in its input. The response of such a sensor to a sudden change or a step input gradually approaches a final equilibrium value. This slow response is due to thermal inertia and varies exponentially relative to time.
A slow sensor may cause instability and reduced accuracy in a control system. By the time the sensor signals the control element to respond to a change in a parameter, the parameter may have already changed to a new value. In other words, the control system lags the change in the parameter. Larger lag is equivalent to less accurate control or instability.
In temperature control systems the accuracy of maintaining a specific temperature depends on the response time of the temperature sensor.
Thermal mass flow meters and controllers use heat transfer and change in temperature of a sensor system as the measuring means for the flow. U.S. Pat. Nos. 4,464,932, 4,984,460 and 5,461,913 are examples of these meters and controllers. Therefore, the measured flow has the same exponential profile as the response of temperature sensor. In a similar way, the accuracy and responsiveness of a flow controller using thermal mass flow meter depends on the response time of the temperature sensor.
A sensor with faster response allows better assessment and control of parameters and processes.